This invention relates generally to additive manufacturing and, more particularly, to apparatus and methods for keeping laser duty cycle within a desired range as part of a direct metal deposition (DMD) system.
Fabrication of three-dimensional metallic components via layer-by-layer laser cladding was first reported in 1978 by Breinan and Kear. In 1982, U.S. Pat. No. 4,323,756 issued to Brown et al., describes a method for the production of bulk rapidly solidified metallic articles of near-net shape, finding particular utility in the fabrication of certain gas turbine engine components including discs and knife-edge air seals. According to the disclosure, multiple thin layers of feedstock are deposited using an energy beam to fuse each layer onto a substrate. The energy source employed may be a laser or an electron beam. The feedstock employed in the practice of the invention may be either a wire or powder material, and this feedstock is applied to the substrate in such a fashion that it passes through the laser beam and fuses to the melted portion of the substrate.
Different technologies have since evolved to improve such processes. U.S. Pat. No. 4,724,299 is directed to a laser spray nozzle assembly including a nozzle body with a housing that forms an annular passage. The housing has an opening coaxial with a passageway, permitting a laser beam to pass therethrough. A cladding powder supply system is operably associated with the passage for supplying cladding powder thereto so that the powder exits the opening coaxial with the beam.
Various groups are now working world-wide on different types of layered manufacturing techniques for fabrication of near-net-shape metallic components. In particular, nozzles of the type described above have been integrated with multi-axis, commercially available CNC machines for the fabrication of 3-dimensional components. U.S. Pat. No. 5,837,960 resides in a method and apparatus for forming articles from materials in particulate form. The materials are melted by a laser beam and deposited at points along a tool path to form an article of the desired shape and dimensions. Preferably the tool path and other parameters of the deposition process are established using computer-aided design and manufacturing techniques. A controller comprised of a digital computer directs movement of a deposition zone along the tool path and provides control signals to adjust apparatus functions, such as the speed at which a deposition head which delivers the laser beam and powder to the deposition zone moves along the tool path.
Most existing techniques, however, are based on open-loop processes requiring either considerable amount of periodic machining or final machining for close dimensional tolerances. Continuous corrective measures during the manufacturing process are necessary to fabricate net shape functional parts with close tolerances and acceptable residual stress. One exception is the system described in U.S. Pat. No. 6,122,564, filed Jun. 30, 1998. This application, the contents of which are incorporated herein by reference, describes a laser-aided, computer-controlled direct-metal deposition, or DMD, system wherein layers of material are applied to a substrate so as to fabricate an object or to provide a cladding layer.
In contrast to previous methodologies, the DMD system is equipped with feedback monitoring to control the dimensions and overall geometry of the fabricated article in accordance with a computer-aided design (CAD) description. The deposition tool path is generated by a computer-aided manufacturing (CAM) system for CNC machining, with post-processing software for deposition, instead of software for removal as in conventional CNC machining. Initial data using an optical feedback loop indicate that it totally eliminates intermediate machining and reduces final machining considerably.
Powder is delivered to the laser melt pool while the deposition head traces its paths across the workpiece. This is how layers are built and stacked one on another. Height control of each layer is necessary to achieve constant and uniform thickness for each layer. Height control is achieved by a feedback system. The workload of the feedback system is measured by how frequently it triggers. Equivalently, the duty cycle sensor measures the usage of the available laser power.
One must be assured that the layers are building to the correct height at ever (x,y,) point of the current plane. Every trigger of the feedback system assures that this is happening. However, every trigger also means that the laser is not being used as efficiently as possible, because ever trigger is equivalent to a momentary reduction in laser power. One source of inefficiency is fluctuations in powder flow rate. Occasionally, the flow rate strays above or below the acceptable range, and if not detected and corrected, this may lead to poor deposition quality and part rework.
If the nozzle is fully clogged or the optical window breaks, the feedback system will receive no trigger input, which is equivalent to a 100% duty cycle. A 100% duty cycle reading for a prolonged period of time would be the stimulus needed to automatically stop the decision. Maintaining quality through on-line feedback control of the powder flow rates will result in more reliable hands-off operation.
Broadly, this invention monitors laser duty cycle in a direct metal deposition (DMD(trademark)) system, and uses the data as input to control another device to keep the duty cycle within a desired range. In the preferred embodiment, the duty cycle is used to control powder flowrate to keep the duty cycle between 75 to 95%. For example, the powder flowrate may be increased or decreased by stepping up or stepping down the angular velocity of the feed-rod.
The duty cycle of the laser is preferably measured by sampling the output signal of the feedback device at a sufficiently high rate, at least twice as fast as the on/off switching speed of the feedback device. The sampled data is stored in a memory buffer, and an algorithm is used to calculate the duty cycle over a period of time specified by the operator. The current duty cycle is preferably displayed on the screen of the operator""s computer along with the values of the periodic measurements stored in the process history database.